(1) Field of the Invention
The present invention relates to combustion control apparatus and method for a spark-ignited internal combustion engine, particularly, in which a stratified combustion drive is carried out.
(2) Description of the Related Art
Two Japanese Patent Application First Publications No. Heisei 11-125131 published on May 11, 1999 and 2000-234542 published on Aug. 29, 2000 exemplify previously proposed combustion control apparatuses for the spark-ignited internal combustion engine in which a nozzle of a fuel injector and a spark plug are exposed to a combustion chamber of each cylinder and a whole engine drive region is divided into two sub-regions depending upon a load imposed on the engine, one drive region being a low load region in which a stratified combustion drive is carried out and the other region being a high load region in which a homogeneous combustion drive is carried out.
It is well known that the spark plug becomes easy to be smoldered because a high temperature cannot be obtained in the stratified combustion drive state as is different from the homogenous combustion drive state. Mechanism of developing the smolder of the spark plug will be described with reference to FIGS. 3A and 3B which show expanded views of a tip portion of the spark plug exposed to the combustion chamber. Most parts of a center electrode 21 of the spark plug is covered with an insulator porcelain 22 and an outer electrode 24 is disposed so as to face against a tip portion 21a of center electrode 21 projected slightly toward a lower end of insulator porcelain 22 with a predetermined gap 23.
In a state in which no smolder state of spark occurs on the spark plug, an air around a part of the outer electrodes 24 which is nearest to outer electrode 24 from tip portion 21a (a lower portion as viewed from FIGS. 3A and 3B) of center electrode 21 is broken down and a spark is emitted. In the stratified combustion drive state which is not easy to obtain a high temperature, a carbon left burned on surfaces of tip portion 21a of center electrode 21 and insulator porcelain 22 is deposited thereon. Since the carbon is a good conductor, a high voltage supplied to center electrode is leaked onto this deposited carbon. Hence a spark begins to be emitted toward a side direction (leftward as viewed from FIG. 3B) if deposited carbon 25 becomes thick, to some degree, as shown in FIG. 3B. This is, as shown in FIG. 3B, the smolder state.
Japanese Patent Application First Publication No. Heisei 11-125131 published on May 11, 1999 exemplifies one of the two previously proposed combustion control apparatuses in which when the secondary voltage is detected and the secondary voltage is determined to be reduced by a value equal to or lower than a predetermined value, a misfire has occurred, the stratified combustion drive is inhibited, and the combustion drive state is switched from the stratified combustion drive to the homogeneous combustion drive. A secondary voltage, viz., a voltage between center electrode 21 and outer electrode 24 is reduced during the occurrence of the smolder state.
However, it is noted that, since a state shown in FIG. 3B indicates an abrupt reduction in the secondary voltage, FIG. 3B shows a state wherein the misfire has occurred. That is to say, the method of detecting the misfire is a detection of the occurrence of the misfire and an immediate state before the misfire cannot be made. Once the combustion chamber falls in a misfire state, it requires a time for the firing state of fuel to be recovered from the misfire.
Since, in this case, there is provided a close relationship between the smoldering and the insulation resistance in the spark plug (a resistance between center electrode 21 and insulator porcelain 22), the insulation resistance of the spark plug may be considered to be adopted as a parameter of a prevention of the misfire in place of the secondary voltage. FIG. 4 shows a model representing influences of the insulation resistance and the secondary voltage against the misfire. As shown in FIG. 4, a resistance value of the insulation resistance of the spark plug reaches to a misfire region at an earlier timing than the resistance corresponding to the secondary voltage with respect to an elapsed time. This means that a method of detecting the misfire from the insulation resistance can detect a state immediate before the occurrence of the misfire with the secondary voltage. As described above, the insulation resistance is superior in detecting the state immediately before the occurrence of the misfire. However, the method of detecting the insulation resistance in an actually engine mounted vehicle is not developed although the insulation resistance can be detected experimentally.
It is, hence, a main object of the present invention to provide combustion control apparatus and method for a spark-ignited internal combustion engine in which the ignition state of fuel during the stratified combustion drive is retained at a state immediately before the smolder occurs to suppress a worsening of a combustion stability at minimum by introducing a counter set to correspond to a drive time duration until a time immediately before the spark is emitted between the outer electrode of the spark plug and deposited carbon on the insulator porcelain, determining whether it is a time at which an inhibit of the stratified combustion drive state on the basis of a value of the counter, and switching forcibly a combustion drive state to the homogenous combustion drive when determining that it is the time at which the inhibit of the stratified combustion drive state. It is another object of the present invention to provide combustion control apparatus and method for a spark-ignited internal combustion engine in which a recovery of the insulation resistance of the spark plug after the inhibit of the stratified combustion drive can be speedily be carried out to return the combustion drive state to the stratified combustion drive by introducing another counter set to correspond to the drive time duration until the insulation resistance of the spark plug is recovered to its original value during the homogenous combustion drive, determining whether it is a time at which the inhibit of the stratified combustion drive is released on the basis of the value of the other counter, and releasing the inhibit of the stratified combustion drive.
This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.